3,5-Di-tert-butyl-4-hydroxybenzoic acid (hereinafter, called as “DBPOB”) is used for synthesizing a wide variety of materials, for example ultraviolet absorber and antioxidant contained in polymer materials such as polypropylene, developer for pressure-sensitive recording paper as well as medical and agricultural chemicals.
WO 2004/031113 (which is incorporated herein by reference) discloses a method for producing DBPOB, which comprises reacting a basic alkali metal compound such as sodium hydroxide with an excess amount of 2,6-di-tert-butylphenol (hereinafter, called as “DTBP”) at a temperature of no lower than 160° C. to give the alkali metal 2,6-di-tert-butylphenolate and reacting said alkali metal 2,6-di-tert-butylphenolate with carbon dioxide to give DBPOB.
This method is advantageous because of the high reaction yield. In addition, the excess DTBP can easily be recovered from the product and reused for synthesizing DBPOB as starting material.
However, the present inventors have found that when the method was conducted with DTBP which was stored for a long time, the expected high yield could not be achieved though the stored DTBP kept the high purity. Moreover, when the method is conducted repeatedly using the excess DTBP separated at the last step as starting material for the next cycle, the yield of DBPOB decreases after 4 th or 5 th cycles. In this case, though there is no problem concerning the purity of the DTBP separated in the last step of each cycle, the yield of DBPOB decreased dependent on the number of the cycles.
Accordingly, there is a need for a method of producing DBPOB with high and stable yield even if DTBP used for the reaction is the one that has been stored for a long time or excess DTBP is recycled more than 4 or 5 times.